Perspectives on field-free spin–orbit torque devices for memory and computing applications

Abstract

The emergence of embedded magnetic random-access memory (MRAM) and its integration in mainstream semiconductor manufacturing technology have created an unprecedented opportunity for engineering computing systems with improved performance, energy efficiency, lower cost, and unconventional computing capabilities. While the initial interest in the existing generation of MRAM—which is based on the spin-transfer torque (STT) effect in ferromagnetic tunnel junctions—was driven by its nonvolatile data retention and lower cost of integration compared to embedded Flash (eFlash), the focus of MRAM research and development efforts is increasingly shifting toward alternative write mechanisms (beyond STT) and new materials (beyond ferromagnets) in recent years. This has been driven by the need for better speed vs density and speed vs endurance trade-offs to make MRAM applicable to a wider range of memory markets, as well as to utilize the potential of MRAM in various unconventional computing architectures that utilize the physics of nanoscale magnets. In this Perspective, we offer an overview of spin–orbit torque (SOT) as one of these beyond-STT write mechanisms for the MRAM devices. We discuss, specifically, the progress in developing SOT-MRAM devices with perpendicular magnetization. Starting from basic symmetry considerations, we discuss the requirement for an in-plane bias magnetic field which has hindered progress in developing practical SOT-MRAM devices. We then discuss several approaches based on structural, magnetic, and chiral symmetry-breaking that have been explored to overcome this limitation and realize bias-field-free SOT-MRAM devices with perpendicular magnetization. We also review the corresponding material- and device-level challenges in each case. We then present a perspective of the potential of these devices for computing and security applications beyond their use in the conventional memory hierarchy.

Current Induced Field‐Free Switching in a Magnetic Insulator with Enhanced Spin‐Orbit Torque

“…In addition, we notice that the field required to eliminate the field-free SOT switching coincides with the enhanced DMI field (Figure 5), suggesting that the chiral symmetry breaking could happen. [46,47] Detailed experiments are needed to clarify such a large enhancement in the interfacial DMI.…”

Electrically Controlled All‐Antiferromagnetic Tunnel Junctions on Silicon with Large Room‐Temperature Magnetoresistance

“…In addition to their significance as a fully functional all-AFM memory on silicon, the field-free operation of these devices also presents a significant advantage over ferromagnetic SOT memory devices, where achieving field-free operation is currently an intensely researched and challenging problem. [47][48][49][50][51][52][53][54][55] There is significant room for further development of AATJ structures based on the general concept presented in this work, e.g., by incorporating other noncollinear antiferromagnetic materials as the free and fixed layers, and by incorporating alternative insulators such as MgO as the tunnel barrier.…”

“…In addition to their significance as a fully functional all‐AFM memory on silicon, the field‐free operation of these devices also presents a significant advantage over ferromagnetic SOT memory devices, where achieving field‐free operation is currently an intensely researched and challenging problem. [ 47–55 ]…”

Progress and Application Perspectives of Voltage‐Controlled Magnetic Tunnel Junctions

“…This would be similar to an approach that has been applied previously to break the inplane symmetry of spin-orbit torque (SOT) MRAM devices, in order to achieve deterministic SOT-induced switching. [142,143] However, the need for simultaneous optimization of EB, PMA, and VCMA within the same material structure presents a significant materials development challenge.…”